Are all the recent big earthquakes connected?



Posted on April 5, 2010 at 2:40 PM

Updated Monday, Apr 5 at 5:29 PM

SEATTLE - In the last 100 years, there are three periods where super earthquakes spiked. We're in the middle of one now. In fact, the magnitude 8.8 Chilean quake on Feb. 27 is ranked by the U.S. Geolgoical Survey as the fifth largest earthquake recorded since the year 1900. The 2004 quake that hit off the coast of Northern Sumatra at magnitude 9.1 ranks third.

The U.S.G.S. says in the last century there are three great periods of large subduction zone quakes where the ocean floor is forced under a nearby continent. Those 20-year periods range between 1905 and 1925, 1950 and 1965 and 2004 to present.  The two largest quakes ever recorded came in the middle, the 1960 9.5 quake in Chile ranks No. 1 and the 9.2 that hit Alaska in 1964 is No. 2.

Are we next?

"There does seem to be some concern," said John Vidale, the head of the Pacific Northwest Seismic Network. "There do seem to be some decades when there are more earthquakes than other decades." 

That does not mean we are next. The last big quake of this type to hit here occurred in January of the year 1700, and is estimated to range between an 8.8 and a 9.2. 

Vidale comes to Seattle from Southern California, where he studied the San Andreas Fault, and looked at how earthquakes can transfer stress to other faults. The southern end of the San Andreas just south of the U.S. border saw the latest big shaker, a magnitude 7.2 that killed at least two people and damaged buildings.  It could have been worse, if it hadn't been for the fact that the quake was centered in a rural area.

"We'd love to know which years are dangerous and which years aren't,"  said Vidale.

But scientists are making connections about earthquakes triggering other quakes.

"We do know that the biggest earthquakes seem to trigger further activity,"  he added. 

But Vidale says that's a easier argument to make when quakes come close together on a map. Yesterday's Mexico quake is on the Laguna Salada fault, which is part of the San Andreas fault system, and aftershocks keep spreading out north into the U.S. and further south into Mexico.

He doubts that February's 8.8 in Chile played a role, nor does he rule it out.

"They're different fault systems with different senses of motion, although they're all along the coast line," he said.

And that's the great unknown. He said stress shift occurs when one earthquake relieves stress in one area but adds stress to another fault.

He also says just plain shaking on one fault can trigger earthquakes on another.  But over distances of thousands of miles is a different challenge.